This invention relates generally to the field of arc fault detectors, and more particularly to an arc fault detector with a filter which prevents false tripping due to dimmer switches.
Arc faults are always accompanied by steps in arc current as the arc strikes, continues, and extinguishes. These steps in current, when differentiated, produce di/dt pulses. Other loads, such as light dimmers, also produce steps in current and the associated di/dt when the dimmer triac switches into conduction. Any current sensor which detects di/dt in the load current waveform responds to both the step in current from an arc fault as well as the step in current from a light dimmer. Arc detectors which are designed to detect series arc faults, which are limited by the load, must by nature be sensitive to low current in the normal load range where loads such as light dimmers can produce arc fault mimicking di/dt.
Briefly stated, an AFCI looks for a variation in the number of load current step pulses, i.e., di/dt pulses, occurring in a succession of power line intervals. A number of di/dt pulses occurring during one cycle of the power wave is compared to a number of di/dt pulses occurring in a subsequent cycle of the power wave. The AFCI activates when the number of times that the di/dt count varies from line cycle to subsequent line cycle exceeds a predetermined number within a predetermined number of line cycles. A counting window within each line cycle preferably extends from approximately 150 degrees after the start of the first half wave cycle to approximately 30 degrees after the start of the following half wave cycle, thereby placing the counting window in the band where light dimmers typically are not designed to operate.
According to an embodiment of the invention, an arc fault protection device protecting a circuit on a power line includes means for detecting di/dt pulses of load current steps, in which a first number of di/dt pulses occurring during a specified interval of a power wave is compared to a second number of di/dt pulses occurring in a subsequent interval of the power wave; and means for interrupting the circuit when a third number of times that the first number differs from the second number exceeds a first determined number in a window consisting of a second determined number of specified intervals of the power wave.
According to an embodiment of the invention, a method for protecting a circuit on a power line includes the steps of (a) detecting di/dt pulses of load current steps; (b) comparing a first number of di/dt pulses occurring during a specified interval of a power wave to a second number of di/dt pulses occurring in a subsequent interval of the power wave; and (c) interrupting the circuit when a third number of times that the first number differs from the second number exceeds a first determined number in a window consisting of a second determined number of specified intervals of the power wave.